A green roof is a vegetated roof that is partially or completely covered with plants. The vegetation is planted over a waterproofing membrane, with additional insulation and drainage layers. Green roofs afford cost-saving and environmental benefits in many areas: they provide increased insulation, extend the life of the roof, increase property values and create natural habitats. They also boost the efficiency of rooftop solar and HVAC systems and reduce air, water, and noise pollution. The two major types of green roofs are extensive and intensive. 

Extensive green roof systems are the thinnest, supporting only the toughest plant species. These systems, ranging in depth from three to six inches, do not have irrigation and are good for growing succulents, such as sedums. Roofs need to support a minimum of 20 pounds per square foot to accommodate them.  Intensive green roof systems are thicker and offer greater growing opportunities.  They also require more maintenance and can take up to a year to become fully established. Intensive systems typically range from six inches to one foot in depth and feature two layers of media, one for drainage and one for growth.  Both layers support more extensive root systems.

Germans have been building green roofs for more than 50 years. The United States has begun to explore this technology and a number of American cities, including New York, Philadelphia, and Chicago, are developing programs to motivate and regulate the green roof industry. The Boston area features green roofs at such institutions as Harvard University and the Massachusetts Institute of Technology. Numerous private residences and commercial buildings in the area are also adding them. A strong motivating factor is the desire to manage stormwater runoff, which will become more and more necessary as the climate changes. Green roofs can reduce annual stormwater runoff by 50%-90% according to research conducted at Penn State University. A 10-year study comparing stormwater runoff between a vegetated roof and traditional asphalt roof in the Boston area found that the vegetated roof retained roughly 50% more stormwater runoff than the asphalt roof.

Vegetated green roofs can be constructed between April and October in New England. The cost to the average homeowner can vary greatly depending on the intricacy of the roof desired, but is generally in the range of $20 and up per square foot.

Jennifer Lawrence works at Apex Green Roofs, Inc., located at 170 School Street in Somerville. For more information call 617-629-2739 or visit ApexGreenRoofs.com.